We observed short spontaneous paroxysmal discharges (100–500 ms) in 23% of slices from 12 U251-GFP glioma-bearing mice in Mg2+-containing artificial cerebrospinal fluid (ACSF) (n = 43), yet these were absent in slices from five sham mice (n = 27) (Fig. 2d). We validated slice viability by recording evoked activity in response to field electrode stimulation in deeper layer 4 (data not shown)